Superposed high frequeny starting circuit for discharge lamp



May 30, 1967 M. R. P. SEIB 3,323,012

SUPERPOSED HIGH FREQUENCY STARTING CIRCUIT FOR DISCHARGE LAMP Filed April 4, 1963 QQQOOOQOQkOQQOQQQOQlOlO QlOQOOQQQOOQOOOQOQQQQOO 0%001 TTOVJ United States Patent 3,323,012 SUPERPOSED IHGH FREQUENCY STARTING CIRCUIT FOR DISCHARGE LAMP Martin Raymond Percival Seib, Hatfield, England, assignor to Caps (Research) Limited, London, England Filed Apr. 4, 1963, Ser. No. 270,580 15 Claims. (Cl. 315-174) This invention relates to circuit arrangements for operating gas discharge lamps.

When commencing operation of a gas discharge lamp the lamp is cold and a high voltage is required to strike an arc in the lamp. Once current begins to flow the voltage necessary to maintain the arc falls and the impedance may therefore be said to fall.

The invention thus includes a circuit arangement for operating a gas discharge lamp comprising a ballast impedance adapted to be connected in series with a gas discharge lamp across a low voltage supply means, and oscillator means effective to supply to the series circuit comprising the lamp and the ballast impedance pulses of voltage of magnitude sufiicient to strike an arc in the lamp, whereby the impedance of the lamp falls and causes damping of the voltage pulses of the oscillator, the current through the lamp being maintained by the low voltage supply means.

In this circuit arrangement the oscillator mean preferably includes an output transformer having a secondary winding connected in series with the lamp and the ballast impedance, whereby the fall in impedance of the lamp causes the gain of the oscillator to fall below unity and thereby efiects damping of the voltage pulses of the scillator.

After a short period the lamp begins to operate continuously at the low voltage and the electrodes become incandescent. At the end of this period the lamp can be run at a still lower applied voltage.

Preferably, therefore, in a circuit arrangement according to the invention set forth above the low voltage supply means includes a first low voltage supply and a second low voltage supply, the arrangement including relay means, time delay means efiective to energise the relay means a predetermined time after the striking of the arc in the lamp, and switch means effective upon energisation of the relay means to disconnect the lamp and the ballast impedance from the first low voltage supply and connect them in series across the second low voltage supply providing a supply voltage of smaller magnitude than the supply voltage from the first low voltage supply but sutficient to maintain current through the lamp after the electrodes have become incandescent.

An embodiment of the invention will now be described with reference to the accompanying drawing which shows a circuit arrangement for operating a mercury are discharge lamp.

With this lamp a voltage of 4,000 volts peak is required to strike the are when the lamp is cold, but when current begins to flow the voltage developed across the lamp falls to about 200 volts peak for a source supplying a current of a few milliamps, or 20 volts for a source supplying several amperes.

A current of about 7 amperes heats the electrodes of the lamp to incandescence in about seconds and in this state they emit electrons freely with the maximum voltage needed to re-strike the are on each half cycle little more than the voltage required to maintain the arc. At the end of this period the electrodes glow red and the light output, the temperature of the vessel, and the vapour pressure inside the vessel are low compared with their values under normal operating conditions. Provided that a current of about 8 amperes is supplied to the lamp the electrical power input exceeds the losses due to radiation and convection so that the lamp runs up towards the normal operating condition at which all of the mercury has evaporated. At this point the voltage across the lamp, and the power input, tends to remain constant and the temperature then rises until the heat losses balance the electrical power supplied.

If the lamp gets too hot the fused quartz of which the lamp vessel is constructed begins to devitrify. The resulting crystalline structure causes both optical obstruction and a mechanical weakening of the vessel.

The circuit arrangement includes an autotransformer 1 which can be connected across the mains supply voltage by means of a mains switch 2 so as to supply current to the remaining components of the circuit.

Connected across a few turns of the autotransformer is a full-wave bridge rectifier 3 which provides an output of unidirectional voltage pulses of frequency equal to twice the mains frequency. This voltage is used to energise an oscillator 4 and the windings 5, 6 and 7 of locking, thermal delay and high voltage relays, respectively, associated with the circuit arrangement. The locking relay controls the operation of first and second locking switches 8 and 9, respectively, the high voltage relay controls a high voltage switch 11 and the thermal delay relay controls a thermal delay switch 12.

The oscillator 4 includes two transistors 13 of the p-n-p type arranged in push-pull relationship. Each transistor has its emitter electrode connected to the positive terminal of the bridge rectifier 3 by means of an emitter resistor 14 and its collector electrode connected to a collector coil 15. The other ends of the collector coils 15 are joined together and to the fixed contact of the first locking switch 8 which is of the on/off type. The movable contact of the first locking switch is connected to the negative output terminal of the bridge 3.

Two resistor networks, each consisting of two resistors 16 and 17 in series, are also connected between the positive output terminal of the rectifier 3 and the fixed contact of the first locking switch 8 and the base electrodes of the transistors are connected to the junctions between the resistors of the respective networks each by means of respective base coils 18. The collector coils 15 and base coils 18 from driving and feedback windings, respectively, of an output transformer 19 of the oscillator.

Connected in parallel between the positive terminal of the bridge 3 and a fixed contact of the thermal delay switch 12, which has two fixed contacts and a movable contact, are the coils 6 and 7 of the thermal delay and high voltage relays, respectively. The movable contact of the thermal delay switch 12 is connected to the junction between the collector coils 15 of the transistors. The other fixed contact of the thermal delay switch 12 is connected to the positive terminal of the bridge 3 by way of the winding 5 of the locking relay. The second locking switch 9 is of the on/off type and has its fixed contact connected to the junction between the locking relay winding 5 and a fixed contact of the thermal delay switch 12 and its movable contact connected to the negative terminal of the bridge 3.

A secondary winding 21 of the output transformer 19 of the oscillator consists of a single winding containing a very large number of turns compared with the total number of turns in the primary windings. One end of the secondary winding 21 is connected to one end of the autotransformer 1 and the other end to a fixed contact 22 of the high voltage switch 11, which is operable by means of the high voltage relay. The high voltage switch 11 also includes two movable contacts of which the first, designated 23 in the drawing, is connected to a tapping in a central position on the autotransformer 1 and the second, designated 24, is connected to one electrode of a gas discharge lamp 25. The other electrode of the lamp 25 is connected in series with a ballast choke 26 to a tapping at the other end of the autotransformer 1. Connected between the fixed contact 22 and the movable contact 24 of the high voltage switch 11 is a resistor 27 of resistance low enough to inhibit the oscillator but high enough to draw negligible current through the secondary winding 21 of the output transformer when not in use. A coil 28 for controlling the operation of an electromagnetic air valve, which in turn controls the supply of cooling air to the discharge lamp 25, is connected between the first movable contact 23 of the high voltage switch and the junction between the discharge lamp 25 and the ballast choke 26.

With the mains switch 2 open and no voltage applied across the autotransformer 1 the high voltage, thermal delay and locking relays are in their de-energized state and the condition of the various switches described above is as follows:

The two movable contacts 23 and 24, respectively, of the high voltage switch 11 are in contact and are not connected to the fixed contact 22; the movable contact of the thermal delay switch 12 is in contact with the fixed contact which is connected to the windings of the high voltage and thermal delay relays 7 and 6, respectively, and the first and second locking switches 8 and 9, respectively, are closed and open, respectively.

Upon closing the mains switch 2 the operation of the circuit is as follows:

The mains voltage is applied to the autotransformer 1 and a small proportion of this is supplied to the input terminals of bridge rectifier 3. Unidirectional voltage pulses are therefore supplied from the output terminals of the rectifier to the oscillator 4, which begins to generate oscillations of high frequency in the output transformer 19. Current also flows from the positive to the negative output terminal of the bridge 3 via the windings of the thermal delay and high voltage relays, the thermal delay switch, and the first locking switch.

The current through the winding of the high voltage relay 7 causps movement of the movable contacts 23 and 24 of the high voltage switch until both are instantaneously in contact with the fixed contact 22, and then a further movement of the first movable contact 23 to leave the second movable contact 24 and the fixed contact 24 only connected together. As a result, the secondary winding 21 of the transformer 19, the discharge lamp 25 and the ballast choke 26 are connected in series across the autotransformer 1.

When in its non-conducting state the discharge lamp 25 has a high impedance. The secondary winding 21 of the output transformer contains a large number of turns and the oscillations generated in the primary windings induce pulses of voltage of large magnitude and high frequency in it, the core of the transformer being driven near to saturation. The high voltage pulses are of suflicient magnitude to strike an arc in the lamp, rendering its impedance low and causing current to flow through it.

This sudden appearance of a low impedance in series with the secondary winding 21 of the output transformer has the effect of damping the oscillations of the oscillator 4 because the change in lamp impedance reduces the oscillator gain below unity. However, the voltage appearing across the autotransformer 1 is sufficient to maintain current through the lamp once the arc has been struck and current is therefore maintained at a fairly low level for about a millisecond while the output transformer core is being driven into saturation. Current through the lamp 25 then increases rapidly up to several amps, being limited primarily by the ballast choke impedance.

If the arc extinguishes on any half cycle of mains voltage before the electrodes of the lamp become incandescent the impedance of the lamp becomes high again and the oscillator is then able to generate a high voltage sufficient to re-strike the are;

At a predetermined time after current begins to flow through the winding of the thermal delay relay 6 the movable contact of the thermal delay switch 12 moves to disconnect the rectifier 3 from the windings of the high voltage and thermal delay relays 7 and 6, respectively, and connect it to the winding of the locking relay 5. This relay is therefore energized and effects opening of the first locking switch 8 and closing of the second locking switch 9.

Current can still flow from the rectifier 3 to the oscillator 4 and to the coil of the locking relay 5 itself.

De-energisation of the winding of the high voltage relay 7 causes the first movable contact 23 of the high voltage switch 11 to engage the second movable contact 24 and both movable contacts then move out of contact with the fixed contact 22. This leaves the ballast choke 26 and the discharge lamp 25 connected in series across approximately half of the winding of the autotransformer 1 which supplies a voltage approximately equal to half of the mains voltage. This reduced voltage is sufficient to run the lamp 25 once the electrodes have become incandescent.

When the circuit is in this normal operating condition the resistor 27 which inhibits oscillation is in parallel with the gap between the fixed contact of the high voltage switch and the two movable contacts. It therefore limits sparking across the gap and erosion of the contacts, which the oscillator output would otherwise have caused.

The winding 28 of the electro-magnetically controlled air valve is now effectively connected in parallel with the lamp 25. At an advanced stage in the run-up, when the lamp arc voltage has risen almost to its normal oper-- ating value the air valve operates, allowing a cooling air supply to reach the lamp. This arrangement is necessary when a lamp, which may nominally be designed for selfcooling in free air conditions, is placed in an enclosure which inhibits convection air currents, which would cause overheating of the lamp. If the cooling air was allowed to play continuously on the lamp it would inhibit the run-up of the lamp, since the cooling air stream would be independent of lamp temperature, unlike the natural convection currents, and the lamp would initially be over-cooled.

I claim:

1. A high-pressure gas-discharge lamp circuit, comprising:

(a) a high-pressure gas-discharge lamp,

(b) a ballast impedance,

(c) means connecting the said lamp in series with said ballast impedance,

((1) oscillator means which are operable from a first low voltage supply means, and whose feedback path includes an output transformer having a secondary winding connected in series with said ballast impedance and said lamp,

(e) the said oscillator means being adapted, in operation, to supply to the series circuit including said lamp and said ballast impedance a high frequency voltage of substantially constant magnitude suflicient to strike an arc in said lamp, whereby the impedance of said lamp decreases, causing the open loop gain of said oscillator means to be reduced below unity and thereby effecting damping of the voltage pulses thereof,

(f) a second low voltage supply means which comprises a first low voltage supply and a second low voltage supply,

(g) means for connecting said series circuit including said lamp and said ballast impedance across said rst low voltage supply of said second low voltage supply means, whereby, after the striking of an arc, current through said lamp is maintained by the said first low voltage supply,

(h) relay means,

(i) time delay means adapted to energize said relay means a predetermined time after switching on said oscillator means to strike an arc in said lamp, and

(j) switch means effective upon energization of said relay means to disconnect said lamp and said ballast impedance from said first low voltage supply of said second low voltage supply means and connect them in series across said second low voltage supply of said second low voltage supply means, which provides a voltage of smaller magnitude than the voltage from said first low voltage supply of said second low voltage supply means but sufficient to maintain current through said lamp after the electrodes thereof have become incandescent.

2. A circuit arrangement as claimed in claim 1, wherein the time delay means include a time delay switch, and the relay means includes a relay winding in series with the time delay switch, the relay means being so arranged that supply of current to the relay winding causes operation of the switch means to connect the lamp and ballast impedance in series with the first voltage supply, and operation of the time delay means after the predetermined time causes opening of the time delay switch, thereby cutting off the supply of current to the relay winding and causing the switch means to connect the lamp and ballast impedance in series with the second low voltage supply.

3. A circuit arrangement as claimed in claim 1, wherein the time delay means includes a time delay relay winding connected in parallel with the relay winding, whereby opening of the time delay switch causes flow of current to both the relay winding and the time delay relay winding to cease.

4. A circuit arrangement as claimed in claim 1, wherein the switch means include a switch having three contacts of which a first contact is connected to a terminal of the second low voltage supply, a second contact is connected to one terminal of the series circuit including the lamp and the ballast impedance, the other terminal of the series circuit being connected to a terminal common to the first and second low voltage supplies, and the third contact is connected to one end of the secondary winding of the output transformer, the other end of the secondary winding being connected to the other terminal of the first low voltage supply, energisation of the switch means by the relay means causing the second and third contacts of the switch to be connected together so that the lamp and ballast impedance are connected in series with the first low voltage supply and energisation of the relay means by the time delay means causing the first and second contacts of the switch to be connected together so that the lamp and ballast impedance are connected in series with the second low voltage supply.

5. A circuit arrangement as claimed in claim 4, wherein the second and third contacts of the switch are connected together by a resistor which prevents arcing between these contacts when the lamp and ballast impedance are connected to the second low voltage supply.

6. A circut arrangement as claimed as claim 4, comprising a control winding for an air valve which winding connects the junction between the lamp and ballast impedance to the first contact of the switch so that the winding is energised when the lamp and ballast impedance are connected to the second voltage supply, energisation of the control winding causing air from the valve to be directed onto the lamp for cooling purposes.

7. A circuit arrangement as claimed in claim 1, wherein the first low voltage supply includes a winding of an input transformer which is connected in series with the lamp and ballast impedance until the said predetermined time after starting of the arc, and the second low voltage supply includes a part of the said winding, which part is connected in series with the lamp and ballast impedance after the said predetermined time.

8. A circuit arrangement as claimed in claim 7, wherein the input transformer is an autotransformer.

9. A circuit arrangement as claimed in claim 7, wherein current for the time delay means and relay means is supplied from a bridge rectifier whose input terminals are connected to the said winding of the input transformer.

10. A circuit arrangement as claimed in claim 9, wherein current for the said oscillator means is supplied from the said bridge rectifier.

11. A circuit arrangement as claimed in claim 1, comprising control means for air supply means arranged to be energised by the time delay means after the said predetermined time, whereby cooling air is directed on to the lamp.

12. A circuit arrangement as claimed in claim 1, wherein the oscillator means is a push-pull oscillator.

13. A circuit arrangement as claimed in claim 12, wherein the oscillator is a transistor oscillator.

14. A high-pressure gas-discharge lamp circuit, comprising:

(-a) a high-pressure, gas-discharge lamp;

(b) a ballast impedance;

(c) means connecting the said lamp in series with said ballast impedance;

(d) oscillator means which are operable from a first low voltage supply means, and whose feedback path includes an output transformer having a secondary winding connected in series with said ballast impedance and said lamp;

(e) the said oscillator means being adapted, in operation, to supply to the series circuit including said lamp and said ballast impedance a high frequency voltage of substantially constant magnitude sufficient to strike an arc in said lamp, whereby the impedance of said lamp decreases, causing the open loop gain of said oscillator means to be reduced below unity and thereby efiecting damping of the voltage pulses thereof;

(f) a second low voltage supply means which comprises a first low voltage supply and a second low voltage supply;

(g) means for initially connecting said series circuit including said lamp and said ballast impedance across said first low voltage supply of said second low voltage supply means, whereby, after the striking of an arc, current through said lamp is maintained by the said first low voltage supply; and

(h) switch means adapted to disconnect said lamp and said ballast impedance from said first low voltage supply of said second low voltage supply means and connect them in series across said second low voltage supply of said second low voltage supply means a predetermined period after switching on said oscillator means to strike an arc in said lamp, which provides a voltage of smaller magnitude than the voltage from said first low voltage supply of said second low voltage supply means but sufficient to maintain current through said lamp after the electrodes thereof have become incandescent.

A high-pressure, gas-discharge lamp circuit, comprising:

(a) a highpressure, gas-discharge lamp;

(b) a ballast impedance;

(c) means connecting said lamp in series with said ballast impedance;

(d) a transistor oscillator which is operable from a first low voltage supply means and whose feedback path includes an output transformer having a secondary winding connected in series with said ballast impedance and said lamp, the said oscillator having a driving coil connected in series with a first electrode of a transistor and a second coil connected in series with a second electrode of said transistor, the coils forming primary windings of said transformer;

(e) the said oscillator being adapted, in operation, to supply to the series circuit including said lamp and said ballast impedance a high frequency voltage of substantially constant magnitude sufficient to strike and are in said lamp, whereby the impedance of said lamp decreases, causing the open loop gain of said oscillator to be reduced below unity and thereby efiecting damping of the voltage pulses thereof;

to strike an arc in said lamp, which provides a voltage of smaller magnitude than the voltage from said first low voltage supply of said second low voltage supply means but sufiicient to maintain current through said lamp after the electrodes thereof have (f) a second low voltage supply means which combecome incandescent.

prises a first low voltage supply and a second low voltage supply;

(g) means for initially connecting said series circuit including said lamp and said ballast impedance across 10 References Cited UNITED STATES PATENTS 2,572,258 9/1951 Goldfield 315-127 said first low voltage supply of said second low volt- 2 784 349 3/1957 Anderson 315 176 age supply means, whereby, after the striking of an 2897403 6/1959 Carboneau; 315 127 arc, current through said lamp is maintained by the 2962667 11/1960 Relatiofi et i 331 i114 581d fi wltage Supply; 2,964,676 12/1960 Davies et al. 315 200.1

(h) swltch means, adapted to disconnect said lamp and 15 3,189 790 6/1965 Nuckons 315 174X said ballast impedance from said first low voltage supply of said second low voltage supply means and con- DAVID J. GALVIN, Primary Examiner. nect them in series across said second low voltage GEORGE WESTBY Examiner.

supply of said second low voltage supply means a predetermined time after switching on said oscillator 20 SRAGOW, Assistant Examine"- 

14. A HIGH-PRESSURE GAS-DISCHARGE LAMP CIRCUIT, COMPRISING: (A) A HIGH-PRESSURE, GAS-DISCHARGE LAMP; (B) A BALLAST IMPEDANCE; (C) MEANS CONNECTING THE SAID LAMP IN SERIES WITH SAID BALLAST IMPEDANCE; (D) OSCILLATOR MEANS WHICH ARE OPERABLE FROM A FIRST LOW VOLTAGE SUPPLY MEANS, AND WHOSE FEEDBACK PATH INCLUDES AN OUTPUT TRANSFORMER HAVING A SECONDARY WINDING CONNECTED IN SERIES WITH SAID BALLAST IMPEDANCE AND SAID LAMP; (E) THE SAID OSCILLATOR MEANS BEING ADAPTED, IN OPERATION, TO SUPPLY TO THE SERIES CIRCUIT INCLUDING SAID LAMP AND SAID BALLAST IMPEDANCE A HIGH FREQUENCY VOLTAGE OF SUBSTANTIALLY CONSTANT MAGNITUDE SUFFICIENT TO STRIKE AN ARC IN SAID LAMP, WHEREBY THE IMPEDANCE OF SAID LAMP DECREASES, CAUSING THE OPEN LOOP GAIN OF SAID OSCILLATOR MEANS TO BE REDUCED BELOW UNITY AND THEREBY EFFECTING DAMPING OF THE VOLTAGE PULSES THEREOF; (F) A SECOND LOW VOLTAGE SUPPLY MEANS WHICH COMPRISES A FIRST LOW VOLTAGE SUPPLY AND A SECOND LOW VOLTAGE SUPPLY; (G) MEANS FOR INITIALLY CONNECTING SAID SERIES CIRCUIT INCLUDING SAID LAMP AND SAID BALLAST IMPEDANCE ACROSS SAID FIRST LOW VOLTAGE SUPPLY OF SAID SECOND LOW VOLTAGE SUPPLY MEANS, WHEREBY, AFTER THE STRIKING OF AN ARC, CURRENT THROUGH SAID LAMP IS MAINTAINED BY THE SAID FIRST LOW VOLTAGE SUPPLY; AND (H) SWITCH MEANS ADAPTED TO DISCONNECT SAID LAMP AND SAID BALLAST IMPEDANCE FROM SAID FIRST LOW VOLTAGE SUPPLY OF SAID SECOND LOW VOLTAGE SUPPLY MEANS AND CONNECT THEM IN SERIES ACROSS SAID SECOND LOW VOLTAGE SUPPLY OF SAID SECOND LOW VOLTAGE SUPPLY MEANS A PREDETERMINED PERIOD AFTER SWITCHING ON SAID OSCILLATOR MEANS TO STRIKE AN ARC IN SAID LAMP, WHICH PROVIDES VOLTAGE OF SMALLER MAGNITUDE THAN THE VOLTAGE FROM SAID FIRST LOW VOLTAGE SUPPLY OF SAID SECOND LOW VOLTAGE SUPPLY MEANS BUT SUFFICIENT TO MAINTAIN CURRENT THROUGH SAID LAMP AFTER THE ELECTRODES THEREOF HAVE BECOME INCANDESCENT. 